复习下C 链表操作（双向循环链表，查找循环节点）

双向循环链表  和 单向循环链表 查找循环节点 思路都是一样。 快慢指针查找法。 理论可参考

c 链表之 快慢指针 查找循环节点

typedef struct Student_Double
{
    char name[10];
    int  point;
    struct Student_Double *preStu;
    struct Student_Double *nextStu;
} StudentDouble;
StudentDouble *  CreateDoubleCircleLink_Table(){
    
    int i = 0;
    StudentDouble *head = NULL;
    head=(StudentDouble *)malloc(sizeof(StudentDouble));
    head->name[0]='';
    head->point = 0;
    head->nextStu = NULL;
    head->preStu = NULL;
    
    //循环节点
    StudentDouble *cirleStu = NULL;
    StudentDouble *temp = NULL;
    StudentDouble *currentNode = head;
    while (i<=9) {
        temp = (StudentDouble *)malloc(sizeof(StudentDouble));
        strncpy(temp->name,"Node",sizeof(temp->name));
        temp->point = i;
        temp->nextStu = NULL;
        temp->preStu = currentNode;
        
        currentNode->nextStu = temp;
        currentNode = temp;
        
        if (i==3) {
            cirleStu = currentNode;
        }
        i++;
    }
    //最后 合并循环节点
    currentNode->nextStu=cirleStu;
    return head;
}
//已知循环节点情况查询循环 链表，验证是否可用
void SelectDoubleLinkTable(StudentDouble *student){
    StudentDouble *next = student->nextStu;
    int i = 0;
    StudentDouble *circleStu = NULL;
    while (next) {
        if (circleStu!=NULL&&next->point == circleStu->point) {
            printf("循环节点%d,结束循环
",next->point);
            break;
        }
        if (i==3) {
            circleStu = next;
        }
        printf("index %d; studentName is %s;  point is %d
",i,next->name,next->point);
        i++;
        next = next->nextStu;
    }


}
//未知情况查询循环节点
StudentDouble * SelectCircleNodeInDoubleLinkTable(StudentDouble *head){
    //快慢指针查询
    StudentDouble *fast = head;
    StudentDouble *slow = head;
    
    while (fast) {
        fast = fast->nextStu->nextStu;
        slow = slow->nextStu;
        
        if (fast==NULL) {//不是循环链表推出
            break;
        }
        if (fast==slow) {//快慢指针相遇
            break;
        }
    }
    if (fast == NULL) {
        printf("该链表 不是循环链表
");
        return NULL;
    }
    
    //查找循环节点
    fast = head;
    while (fast!=slow) {
        fast=fast->nextStu;
        slow=slow->nextStu;
    }
    printf("=====找到循环链表循环节点为%d
",fast->point);
    return fast;
}
int main(void){
    char sf[15];
  //创建双向循环链表
    StudentDouble *head = NULL;

    printf("创建双向循环链表Y|N
");
    scanf("%s",sf);
    if (strcmp(sf,"Y")==0) {
        head = CreateDoubleCircleLink_Table();
    }
    printf("已知情况查询循环链表Y|N 
");
    scanf("%s",sf);
    if (strcmp(sf,"Y")==0) {
        SelectDoubleLinkTable(head);
    }
    printf("未知情况查询循环链表Y|N 
");
    scanf("%s",sf);
    if (strcmp(sf,"Y")==0) {
        SelectCircleNodeInDoubleLinkTable(head);
    }
    return 0;
}